are the same bits that were introduced in Section 2.4.3 and later applied in Chapter 6,
and to a lesser extent in Chapter 7. In Chapter 6, Digital Networks, the primary purpose
of those bits was to signal the distant end value of the voltage level of an analog channel
at some moment in time. Here we will be assembling bit groupings that will represent
letters of the alphabet, numerical digits 0 through 9, punctuation, graphic symbols, or just
operational bit sequences that are necessary to make the data network operate with little
or no ostensible outward meaning to us.
From old-time telegraphy the terminology has migrated to data communications. A
mark is a binary 1 and a space is a binary 0. A space or 0 is represented by a positive-
going voltage, and a mark or 1 is represented by a negative-going voltage. (Now
getting confused. When I was growing up in the industry, a 1 or mark was a positive-going
voltage, and so forth.)
REMOVING AMBIGUITY: BINARY CONVENTION
To remove ambiguity of the various ways we can express a 1 and a 0, CCITT in Rec. V.1
(Ref. 2) states clearly how to represent a 1 and a 0. This is summarized in Table 10.1,
with several additions from other sources. Table 10.1 defines the sense of transmission
so that the mark and space, the 1 and 0, respectively, will not be inverted. Inversion can
take place by just changing the voltage polarity. We call it reversing the sense. Some data
engineers often refer to such a table as a "table of mark-space convention."
Written information must be coded before it can be transmitted over a data network. One
bit carries very little information. There are only those two possibilities: the 1 and the 0.
It serves good use for supervisory signaling where a telephone line could only be in one
of two states. It is either idle or busy. As a minimum we would like to transmit every
letter of the alphabet and the 10 basic decimal digits plus some control characters, such
as a space and hard/soft return, and some punctuation.
Suppose we join two bits together for transmission. This generates four possible bit
Equivalent Binary Designations: Summary of Equivalence
Mark or marking
Space or spacing
Hole (in paper tape)
No hole (in paper tape)
Tone on (amplitude modulation)
Low frequency (frequency shift keying)
Inversion of phase
No phase inversion (differential phase shift keying)
Opposite to reference phase
To a certain extent, this is a review of the argument presented in Section 6.2.3.